stm32: enable flash bank size override

It has been seen on some stm32 targets that the flash size register that
is probed by the driver may contain an invalid size.

This change enables the user to override the probed value.

Change-Id: I09359e59a96f9133d3d939670957d32a830a944e
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1132
Tested-by: jenkins
Reviewed-by: Johan Almquist <johan.almquist@assaabloy.com>
stm32: add support for the STM32Lx 384kb dual bank flash

This update adds support for the STM32Lx 384kb dual bank flash. Previously there was a problem when writing an
image that was larger than 192Kb. That lead to openocd printing out two error messages like
"Error: access denied / write protected" and "Error: invalid program address". The reason was that the stm32lx
driver tried to write half pages which overlapped into the next flash bank.
A new configuration file stm32lx_dual_bank.cfg can be used for stm32lx chips with dual bank flash (256kb or 384kb devices).
A sanity check was added for probed flash size values to fix the issue seen on some ST samples that answered incorrectly.

Change-Id: I69e25131983d88613be8606b438f98870c5f1e52
Signed-off-by: Johan Almquist <johan.almquist@assaabloy.com>
Reviewed-on: http://openocd.zylin.com/1125
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>